2. Topics Covered
► Definitions
► Characteristics
► Comparison between Routers and Layer 3 Switches
► Functionality
► The Benefits of Layer 3 Switching
► Cost
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3. Router
► A router is a computer
networking device that
buffers and forwards data
packets across an
internetwork toward their
destinations, through a
process known as routing.
► Routing occurs at layer 3
(Network layer )
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4. Network Switches
► A network switch
is a networking device
that performs
transparent
connection of
multiple network
segments with
forwarding based on
MAC addresses at up
to the speed of the
hardware.
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5. Layer 3 Switches
► A Layer 3 switch is a high-performance device for network routing. It is
relatively new term, was conceived as a technology to improve on the
performance of routers used in large local area networks (LANs).
► It can support the same routing protocols as network routers do. Both
inspect incoming packets and make dynamic routing decisions based on
the source and destination addresses inside.
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6. Characteristic
► It is designed to handle high-performance LAN traffic, so
Layer 3 switch can be placed anywhere within a network
core or backbone, easily and cost-effectively replacing the
traditional backbone router.
► The switches run routing protocols, such as open shortest
path first (OSPF) or routing information protocol (RIP), to
communicate with other Layer 3 switches or routers and to
build their routing/forwarding tables. These tables are
looked up to determine the route for an incoming packet
► a Layer 3 switch can reprogram the hardware dynamically
with the current Layer 3 routing information. This is what
allows much faster packet processing.
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7. Similarities
A Layer 3 switch does everything to a packet that a
traditional router does:
► Determines forwarding path based on Layer 3
information
► Validates the integrity of the Layer 3 header via
checksum
► Verifies packet expiration and updates accordingly
► Processes and responds to any option information
► Updates forwarding statistics in the Management
Information Base (MIB)
► Applies security controls if required
► Both types of boxes share a similar appearance
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8. Differences
► The key difference between Layer 3 switches and routers
lies in the hardware technology used to build the unit. The
hardware inside a Layer 3 switch merges that of traditional
switches and routers, replacing some of a router's software
logic with hardware to offer better performance in some
situations.
In general-purpose routers, packet switching takes place
using a microprocessor, whereas a Layer 3 switch
performs this using application-specific integrated
circuit (ASIC) hardware.
Another differentiating feature between a router and a
Layer 3 switch is the number of ports to which
individual terminal devices can be connected. A Layer 3
switch usually has a significantly greater port density.
► Layer 3 switches often cost less than traditional routers
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9. Functionality
► The principle of a Layer 3 is "route once, switch many".
A Layer 3 switch can actually store MAC to IP
Address associations for the source and destination of
the packet.
when there is a packet with a new source destination
pair, it is routed (which means that the routing table
is referred to for determining the best path ) which is
time consuming, and this source destination pair
information is stored in the router cache.
when the next packet arrives with the same source
destination pair, no routing lookup is needed and the
packet is directly switched (which is much faster).
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10. The benefits of Layer 3 switching
The benefits of layer 3 switching include the following:
► Hardware-based packet forwarding
► High-performance packet switching
► High-speed scalability
► Low latency
Latency is the time taken for a packet of data from the
sending application to the receiving application. This
includes both transit time over the network, and
computer processing time at both the source and
destination computers.
► Lower per-port cost
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11. Cont…
► Flow accounting
► Security
► Quality of service
( QoS) refers to control mechanisms that can provide
different priority to different users or data flows, or
guarantee a certain level of performance to a data flow
in accordance with requests from the application
program
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12. $$$ Cost $$$
► Cost Savings
A traditional router may run U.S. $8,000 to $10,000 per
Fast Ethernet interface, while a Layer 3 switch costs less
than U.S. $1,000 per port.
Layer 3 switching is cheap because Layer 3 switches are
targeted specifically for inter-VLAN routing, where only
Ethernet access technologies are used in high densities
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13. What is VLAN
► A virtual LAN, is a method of creating independent
logical networks within a physical network. Several VLANs
can co-exist within such a network. This helps in reducing
the broadcast domain and aids in network administration
by separating logical segments of a LAN (like company
departments) that should not exchange data using a LAN
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15. Do Layer 3 Switches Completely
Eliminate Need for the Traditional
Router
► Routers are still needed, especially where connections to
the wide area are required. Layer 3 switches may still
connect to such routers to learn their tables and route
packets to them when these packets need to be sent over
the WAN.
► The switches will be very effective on the workgroup and
the backbone within an enterprise, but most likely will not
replace the router at the edge of the WAN .
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16. ► Routers perform numerous other functions like
filtering with access lists,
inter-Autonomous System (AS) routing with protocols
such as the Border Gateway Protocol (BGP)
Some Layer 3 switches may completely replace the need for
a router if they can provide all these functions
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